1. Field of the Invention
The present invention relates to a method and apparatus for producing hydrophobic silica fine powder which can be used as a thickener for coatings, adhesives and synthetic resins, as a reinforcement for plastics, and to improve flowability in toners for copiers.
2. Prior Art
Pyrogenic silica (silicon dioxide) is very fine, having a particle size of about 5 to 50 nm. Because it is difficult to collect in this form, it is agglomerated, then collected. The agglomerated silica contains a high concentration of chlorine, and must therefore be deacidified. Deacidification is generally carried out in a fluidization vessel. When agglomerated silica is deacidified, only a small amount of silica flies out of the fluidization vessel together with waste gases. However, when the silica is treated with a hydrophobizing agent, due to breakup of the agglomerate by such treatment, at least several times more treated silica flies out of the fluidization vessel together with waste gases than when agglomerated silica is directly deacidified. The presence of such fugitive treated silica in the waste gases leads to a number of practical obstacles when the waste gases are treated with a scrubber, such as the formation of foam, which cannot be easily removed with filters.
It is therefore an object of the invention to provide a method and apparatus for producing hydrophobic silica fine powder by hydrophobizing agglomerated silica with an organohalosilane in a fluidization vessel, which are designed such that a part of the treated silica which flies out of the vessel together with waste gases can be reliably recovered without complicating the apparatus or process control.
In studies where we installed cyclones and bag filters to recover silica that had flown out from fluidization vessels and examined the degree of fly-out based on the amount of silica recovered, we found the fly-out ratio to be 0.3 to 0.5% when conventional pyrogenic silica is deacidified in a fluidization vessel, and 4 to 15% when such silica is first treated with a hydrophobizing agent then deacidified. While the shape of the equipment and the fluidizing conditions also have an effect on the fly-out ratio, this large difference appears to be attributable to the breakup of agglomerates in hydrophobizing treatment, which leads to easier fly-out than when the silica is subjected only to deacidification. Recovery of the fugitive silica is thus necessary to improve product yield and alleviate the burden on waste gas treatment.
However, unreacted organohalosilane (referred to hereinafter as xe2x80x9csilanexe2x80x9d) hydrophobizing agent present in the waste gases forms a gel or oil due to the condensation of moisture in the waste gases, which can lead to the clogging and obstruction of equipment and lines. On measuring and studying the temperature at various places in the exhaust system, we have found that, if the temperature of the equipment and waste gases is maintained at 100xc2x0 C. or higher, the moisture present in the waste gases does not condense and undesirable products such as gels or oils due to moisture and unreacted silane do not form. In particular, the absence of gel or oil formation on the filter fabric in a bag filter keeps the filter fabric free of clogging, making it possible to carry out continuous operation.
The degree of fly-out also varies with the flow conditions. In hydrophobizing treatment, a high concentration of chlorine is generally present in the gas, creating a need for subsequent deacidification. However, it is more effective to carry out hydrophobizing treatment and deacidification separately, in which case the presence or absence of moisture comes to have an effect on flow of the material during deacidification. An investigation on the level of water showed us that material fluidization is poor in the absence of moisture, but that the addition of even a very small amount of water to the fluidizing gas improves the flow state and reduces fly-out. Less fly-out makes it possible to lower the burden on cyclones and especially bag filters.
We thus discovered that by holding down fly-out and maintaining the temperature of the cyclone and bag filter at 100xc2x0 C. or higher, essentially 100% of fugitive silica can be recovered.
Accordingly, the invention provides a method for producing hydrophobic silica fine powder. A silane compound is pyrolyzed to form a silica fine powder. The silica fine powder is then hydrophobized with an organohalosilane in a fluidization vessel, giving hydrophobized silica fine powder which is collected. The hydrophobized silica fine powder which flies out of the fluidization vessel is collected with a cyclone and bag filter which are held at a temperature of 100 to 500xc2x0 C.
In a preferred embodiment, the fluidization vessel includes a hydrophobizing section where the silica fine powder is hydrophobized and a deacidifying section where deacidification is carried out following hydrophobization. Deacidification is preferably carried out in the deacidifying section by adding 0.1 to 1 vol % of water to a fluidizing gas.
The invention also provides an apparatus for producing hydrophobic silica fine powder, which apparatus includes a means for pyrolyzing a silane compound to form silica fine powder, a means for agglomerating the silica fine powder, a first cyclone and a first bag filter for collecting the agglomerated silica fine powder, a fluidization vessel having a hydrophobizing section for hydrophobizing the collected silica fine powder, and a second cyclone and a second bag filter for collecting the hydrophobic silica fine powder which flies out of the fluidization vessel. The second cyclone and the second filter can each be held at a temperature of 100 to 500xc2x0 C.
The advantages of the invention are as follows. When silane is flame-hydrolyzed to form silica fine powder, and the silica is then hydrophobized in a fluidization vessel using a hydrophobizing agent such as an organohalosilane, the amount of silica that flies out of the vessel into the waste gases is greater than when hydrophobizing treatment is not carried out. During recovery of the silica in the waste gases, the condensation of moisture in the waste gases converts unreacted organohalosilane hydrophobizing agent which emerges together with the waste gases into an undesirable gel or oil. In the method and apparatus of the invention, by maintaining the cyclone and bag filter used as the recovery devices at a temperature of at least 100xc2x0 C., no organohalosilane gel or oil forms and thus no clogging of lines or bag filter pores occurs, making continuous operation possible. Moreover, the inventive method and apparatus enable essentially 100% recovery of fugitive silica, resulting in a higher product yield. An additional advantage is that, even when the waste gases are treated with a scrubber, there is little if any fugitive silica-induced formation of foam, which cannot be easily removed with filters. This greatly alleviates the burden on waste gas and wastewater treatment.